An external server is not really more
secure; the data can be encrypted
to the server. Local AAA may be
more reliable, depending on your
network to the external server and the
server itself (another failure point).
If you have multiple router's,
maintenance is likely easier with AAA -
one change and it applies everywhere.
In general, if your network (# of
devices, # of administrators, # of
users, etc) grows handling router
access via an external AAA server is
usually going to be better for the
long run.
